Some communication systems allow the user of a device, such as a personal computer, to conduct voice or video calls over a packet-based computer network such as the Internet as well as conventional circuit switched networks such as GSM and PSTN communication networks. Such communication systems include voice or video over internet protocol (VoIP) systems. These VoIP systems are beneficial to the user as they are often of significantly lower cost to use than the conventional fixed line (PSTN) or mobile cellular (GSM) networks. This may particularly be the case for long-distance communication. To use a VoIP system, the user installs and executes client software on their device. The client software sets up the VoIP connections as well as providing other functions such as registration and authentication. In addition to voice communication, the client may also set up connections for other communication media such as instant messaging (“IM”), SMS messaging, file transfer and voicemail.
With increasing mobile bandwidths, there is increasing interest in providing packet-based voice and video calls via client applications running on user terminals such as Internet-enabled mobile phones. These user terminals comprise network interfaces 224 such as short-range RF transceivers operating on one or more unlicensed bands for accessing the Internet via wireless access points (e.g. of Wi-Fi access points of WLAN networks), and/or cellular transceivers operating on one or more licensed bands for accessing the Internet via a packet-based service of a cellular network such as GPRS (General Packet Radio Service) or HSPA (High Speed Packet Access).